Stellar Disks in Aquarius Dark Matter Haloes

Abstract

We investigate the gravitational interactions between live stellar disks and their dark matter halos, using LCDM haloes similar in mass to that of the Milky Way taken from the Aquarius Project. We introduce the stellar disks by first allowing the haloes to respond to the influence of a growing rigid disk potential from z = 1.3 to z = 1.0. The rigid potential is then replaced with star particles which evolve self-consistently with the dark matter particles until z = 0.0. Regardless of the initial orientation of the disk, the inner parts of the haloes contract and change from prolate to oblate as the disk grows to its full size. When the disk normal is initially aligned with the major axis of the halo at z=1.3, the length of the major axis contracts and becomes the minor axis by z=1.0. Six out of the eight disks in our main set of simulations form bars, and five of the six bars experience a buckling instability that results in a sudden jump in the vertical stellar velocity dispersion and an accompanying drop in the m=2 Fourier amplitude of the disk surface density. The bars are not destroyed by the buckling but continue to grow until the present day. Bars are largely absent when the disk mass is reduced by a factor of two or more; the relative disk-to-halo mass is therefore a primary factor in bar formation and evolution. A subset of the disks is warped at the outskirts and contains prominent non-coplanar material with a ring-like structure. Many disks reorient by large angles between z=1 and z=0, following a coherent reorientation of their inner haloes. Larger reorientations produce more strongly warped disks, suggesting a tight link between the two phenomena. The origins of bars and warps appear independent: some disks with strong bars show no disturbances at the outskirts, while the disks with the weakest bars show severe warps.